Buried semiconductor laser and method for manufacturing the same

ABSTRACT

A buried semiconductor laser exhibiting a reduced dislocation of a contact layer is achieved. A buried semiconductor laser, comprising: an n-type indium phosphide (InP) substrate; an active layer disposed on the n-type InP substrate; block layers provided so as to bilaterally disposed on both sides of the active layer; a clad layer provided so as to cover the active layer and the block layers; and a p-type gallium indium arsenide (InGaAs) contact layer provided on the clad layer, wherein the p-type InGaAs contact layer has a compressive strain.

This application is based on Japanese patent application No.2006-333,261, the content of which is incorporated hereinto byreference.

BACKGROUND

1. Technical Field

The present invention relates to a buried semiconductor laser and amethod for manufacturing the same. Specifically, the present inventionrelates to a structure of a p-type gallium indium arsenide (InGaAs)contact layer in a buried semiconductor laser of group III-V element,employed for optical communications, and a method for manufacturing thesame.

2. Related Art

In an optical transmission system that achieves a communication withhigher capacity and higher rate, semiconductor lasers employed as lightsources are mainly composed of indium phosphide (InP)-containing, groupIII-V compound semiconductors. Lower threshold current and higherefficiency are required for an active layer in a semiconductor laser, sothat a strained quantum-well structure is used in order to improve suchcharacteristics. For example, a technology for achieving a lowerthreshold current by applying compressive strain of 1.53% to a welllayer is disclosed in Electronics Letters, vol. 26 (1990), p.p. 465-467.However, when a strain larger than a critical strain is applied to aquantum well layer, a defect such as dislocation is generated in anemission layer, considerably deteriorating laser characteristics. Tosolve the problem, Japanese Patent Laid-Open No. H4-22,185 (1996)discloses a semiconductor optical device having a quantum well layercomposed of a multiple-layered structure with compressive strain layersand tensile strain layers, so that an average strain is reduced to alevel of not higher than a threshold strain, thereby achieving animproved performance of a semiconductor optical device.

Since a larger strain to the active layer is applied to thesemiconductor devices described above at a level up to the limit of ofthe material property, the devices are structurally vulnerable by astress generated in the whole laser device. In general, a buriedstructure, which is manufactured by forming pn-buried type or higherresistance buried-type current-blocking structures are formed on bothsides of the active layer and then filling the whole current-blockingstructures with an InP clad layer, is also employed in order to reduce aleakage current from the semiconductor laser around the active layer atwider range of temperatures to achieve a faster operation of the device.In such buried layer, it is required to control a strain for the purposeof avoiding a generation of a dislocation extending to the active layer.

In a semiconductor light emission device described in Japanese PatentLaid-Open No. H11-87,764 (1999), a multiple-layered semiconductor layerincluding a buffer layer, a first clad layer, an active layer, a secondclad layer and a cap layer is formed on an n-InP compound semiconductorsubstrate. The semiconductor light emission device described in JapanesePatent Laid-Open No. H11-87,764 is configured that at least one of abuffer layer and a cap layer has a tensile strain, so that improvementsin device characteristics such as reduction in threshold current wouldbe achieved.

On the other hand, a semiconductor laser device having a contact layeris disclosed in Japanese Patent Laid-Open No. 2004-95,975. Thesemiconductor laser device disclosed in Japanese Patent Laid-Open No.2004-95,975 is designed that a different strain level between in a firstactive layer and in a second active layer, so as to suitably control again peak wave-length. The semiconductor laser device described inJapanese Patent Laid-Open No. 2004-95,975 is provided with a strainadjustment film to control a strain in the active layer. A growth of anInGaAs contact layer described in Japanese Patent Laid-Open No.2004-95,975 is conducted concurrently with a growth of the active layer,which achieves a flat geometry. Since a strain in the active layer iscreated by utilizing film stresses generated in an insulating film andan electrode film disposed on the InGaAs contact layer in JapanesePatent Laid-Open No. 2004-95,975, the InGaAs contact layer does notexhibit a strain.

In a structure of a buried semiconductor laser employing an n-type InPsubstrate, a p-type InGaAs contact layer having a carrier density of1×10¹⁹ cm⁻³ or higher is required to be disposed between a p-sidemetallic electrode and a p-type InP clad layer, for reducing a drivingcurrent of semiconductor laser. Since such structure of the buriedsemiconductor laser employing the n-type InP substrate is configuredthat the pnp buried layers or the high resistance buried layers areselectively grown on both sides of the active layer, the whole structureis filled with the p-type InP clad layer, and then the p-type InGaAscontact layer is grown, the portion of the resultant contact layerimmediately above the active layer is not completely flat and aninclined portion is remained.

When the InGaAs contact layer is in lattice match with InP at a flatsection that is sufficiently remote by 50 μm or further from theinclined portion of the p-type InGaAs contact layer immediately abovethe active layer in such structure of the buried semiconductor laseremploying the n-type InP substrate, a tensile strain toward the InP cladlayer is applied at the inclined portion of the p-type InGaAs contactlayer immediately above the active layer. Such tensile strain generatedat the inclined portion of the p-type InGaAs contact layer immediatelyabove the active layer may possibly induce a generation of a defect suchas dislocation and the like, causing a concern for adversely affecting areliability of the semiconductor device.

According to detailed investigations by the present inventors, it wasfound in such structure of the buried semiconductor laser employing then-type InP substrate that, although the p-type InGaAs contact layer inthe flat section, which is sufficiently remote by 50 μm or further fromthe inclined portion immediately above the active layer, is in acondition of lattice match on the InP substrate, a larger tensile strainof about 0.3 to 0.4% at a maximum is applied to the inclined portionimmediately above the active layer. It is considered that the reason isa difference in uptake efficiency for atomic In and atomic Ga betweenthe inclined portion and the flat portion. In general, the atomic uptakeefficiency between the inclined portion and the flat portion is variedsomewhat by growth conditions of the p-type InGaAs layer, but it isdifficult to obtain the same composition for the inclined portion andfor the flat portion. A tensile strain in the portion of the p-typeInGaAs contact layer in the inclined portion immediately above theactive layer may induce a generation of a defect such as dislocation andthe like, leading to adversely affecting a reliability of thesemiconductor device.

The present invention is made on the basis of the above-describedcircumstances, and is directed to a semiconductor device exhibiting animproved reliability by inhibiting a generation of a defect such asdislocation and the like.

SUMMARY

According to one aspect of the present invention, there is provided aburied semiconductor laser, comprising: an n-type indium phosphide (InP)substrate; an active layer disposed on the n-type InP substrate; blocklayers bilaterally provided on both sides of the active layer; a cladlayer provided over the active layer and the block layers; and a p-typegallium indium arsenide (InGaAs) contact layer provided over the cladlayer, wherein the p-type InGaAs contact layer has a compressive strain.A strain to be applied to the contact layer can be suitably controlledto reduce a tensile strain in the whole contact layer, so that a contactlayer, which exhibits reduced chances of a generation of a dislocation,can be obtained.

According to another aspect of the present invention, there is provideda method for manufacturing a buried semiconductor laser, comprising:forming an active layer an on n-type InP substrate; forming block layerson both sides of the active layer; forming a clad layer over the activelayer and the block layer; and forming a p-type InGaAs contact layerover the clad layer, wherein the forming the p-type InGaAs contact layerincludes forming the p-type InGaAs contact layer having a compressivestrain.

The buried semiconductor laser according to the present invention mayalternatively have a configuration, in which the p-type InGaAs contactlayer includes a flat portion remote from a portion thereof immediatelyabove the active layer and an inclined portion immediately above theactive layer, and furthermore the flat portion of the p-type InGaAscontact layer has a compressive strain that is not higher than acritical strain, and the inclined portion of the p-type InGaAs contactlayer has a tensile strain that is not higher than a critical strain.

A composition of the flat portion is corrected toward a side of thecompressive strain in the growth of the layer, so that the tensilestrain in the inclined portion is reduced, thereby avoiding a generationof a defect such as dislocation and the like.

The buried semiconductor laser according to the present invention mayalternatively have another configuration, in which the flat portion ofthe p-type InGaAs contact layer has a layer thickness of 0.3 μm, and thecompressive strain of the flat portion is within a range of from 0.1% to0.2%.

A p-type InGaAs contact layer having a thickness of 0.3 μm, which is asufficient thickness for ensuring a diffusion length after an alloyprocess with an electrode metal, which is a necessary process forproviding better ohmic contact with the electrode metal, was employed toconduct an investigation for a critical strain level on the basis of amodel proposed by J. W. Matthews and A. E. Blakeslee (J. Crystal Growth,27 (1974), J. W. Matthews and A. E. Blakeslee, 118-125), and the resultsof the investigation show that the compressive strain in the flatportion within a range of from 0.1% to 0.2% would provides a formationof the p-type InGaAs contact layer having the flat portion and theinclined portion, each of which has a thickness that is not higher thana critical strain, thereby reducing a generation of a defect such asdislocation and the like. The thickness of the p-type InGaAs contactlayer of equal to or smaller than 0.3 μm provides further largertolerance for the compressive strain composition of the p-type InGaAscontact layer.

The buried semiconductor laser according to the present invention mayalternatively have further configuration, in which the p-type InGaAscontact layer is composed of a superlattice structure of a p-type indiumarsenide (InAs) layer with a p-type gallium arsenide (GaAs) layer, andwherein the p-type InAs layer and the p-type GaAs layer have averagecompressive strains within a range of from 0.1% to 0.2% in the flatportion.

While the p-type InAs layer and the p-type GaAs layer have latticemisfits for the n-type InP substrate of +3.2% and −3.7%, respectively,an average compressive strain in the flat portion of the superlatticestructure is controlled to be within a range of from 0.1% to 0.2%, suchthat the contact layer of p-type InAs/GaAs superlattice structure havingthe flat portion and the inclined portion, each of which has a thicknessthat is not higher than a critical strain, is obtained.

According to the present invention, the buried semiconductor laserexhibiting a reduced dislocation of the contact layer is achieved.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other objects, advantages and features of the presentinvention will be more apparent from the following description ofcertain preferred embodiments taken in conjunction with the accompanyingdrawings, in which:

FIGS. 1A and 1B are a cross-sectional view of a buried semiconductorlaser according to the first embodiment of the present invention;

FIG. 2 is a graph showing a distribution of a strain in a p-type InGaAslayer of FIG. 1;

FIG. 3 is a cross-sectional view of a buried semiconductor laseraccording to the second embodiment of the present invention;

FIG. 4A to FIG. 4F are cross-sectional views of a buried semiconductorlaser, showing a method for manufacturing the buried semiconductor laseraccording to the first embodiment of the present invention; and

FIG. 5 is a graph showing a relationship of a composition of the p-typeInGaAs layer with a strain.

DETAILED DESCRIPTION

The invention will be now described herein with reference toillustrative embodiments. Those skilled in the art will recognize thatmany alternative embodiments can be accomplished using the teachings ofthe present invention and that the invention is not limited to theembodiments illustrated for explanatory purposed.

Preferable exemplary implementations of buried semiconductor lasersaccording to the present invention will be described in reference to theannexed figures. In all figures, an identical numeral is assigned to anelement commonly appeared in the description of the present invention inreference to the figures, and the detailed description thereof will notbe repeated.

A buried semiconductor of an embodiment of the present inventionincludes an n-type InP substrate, an active layer disposed on the n-typeInP substrate, block layers bilaterally provided on both sides of theactive layer, a clad layer provided over the active layer and the blocklayers; and a p-type gallium indium arsenide (InGaAs) contact layer onthe clad layer. When the p-type InGaAs crystal is intended to be grownon the n-type InP substrate, a strain (e) in the p-type InGaAs contactlayer is determined by gallium (Ga) composition (x). Such relationshipwill be described as follows.

Lattice constants of InP, GaAs, InAs and InGaAs are written as a(InP),a(GaAs), a(InAs) and a(In_(1-x)Ga_(x)As), respectively, and

a(InP)=5.8688 angstroms;

a(GaAs)=5.6533 angstroms;

a(InAs)=6.0584 angstroms; and

a(In_(1−x)Ga_(x)As)=xa(GaAs)+(1−x)a(InAs)=6.0584−0.4051x   (1),

according to “HIKARI TSUSHIN SOSHI KOGAKU” (Optical Communication DeviceEngineering), Kogaku Tosho Co., Ltd.), pp. 75. Therefore, relationshipof strain (ε) with Ga composition (x) is presented by the followingformula 2:

ε(%)=[a(In1−xGaxAs)−a(InP)]/a(InP)=3.2306−6.9026x   (2).

A graph related to the above-described formula 2 is shown in FIG. 5,with ordinate for strain ε (%), abscissa for Ga composition x. Apositive strain ε represents a compressive strain, and a negative strainε represents a tensile strain. For example, when the p-type InGaAscontact layer has no strain (ε=0), or more specifically, is in latticematch with the n-type InP substrate, the composition of the InGaAscontact layer is obtained as In_(0.53)Ga_(0.47)As (x=0.47) according toformula 2. In the present invention, the composition of the InGaAscontact layer is determined by employing the above-described formula 2.

First Embodiment

FIG. 1A illustrates a structure of a buried semiconductor laseraccording to the first embodiment of the present invention. Thesemiconductor device of the present embodiment includes an n-type InPsubstrate 11, an active layer 17 as a light emitting region, a p-typecurrent block layer 18, an n-type current block layer 19, a p-type cladlayer 20, and a p-type InGaAs contact layer 21.

The block layers 18 and 19 are provided on both sides of the activelayer 17. In other words, the active layer 17 is disposed between a pairof the block layers 18 and 19. The p-type clad layer 20 is disposed soas to cover the active layer 17 and the block layers 18 and 19.

FIG. 1B illustrates an enlarged view of a structure around the activelayer 17 showed in FIG. 1A. A mesa structure including an n-type InPclad layer 12, n-side InGaAsP guide layer 13, the active layer 17, ap-side InGaAsP guide layer 15, and a p-type InP clad layer 16 isprovided on the n-type InP substrate 11. The active layer 17 has aquantum well structure.

The p-type InGaAs contact layer 21 is provided to cover the p-type InPclad layer 20. The contact layer 21 has an inclined portion in a regionimmediately above the active layers, and a flat portion in a regionremote from the region immediately above the active layer. While theinclined portion is illustrated in FIG. 1 to have a configuration wherea bottom surface has a U-shaped cross section, the configuration thereof is not limited to such configuration, and for example, aconfiguration having a flat horseshoe cross section of the bottomsurface may be employed.

A strain in the portion of the p-type InGaAs contact layer 21immediately above the active layers (inclined portion) over the p-typeInP clad layer is evaluated in detail by utilizing μ-X-ray emitted froma light source of a synchrotron radiation. Such μ-X-ray evaluation isconducted by utilizing a light beam having a beam diameter of about 1μm. FIG. 2 is graph showing a strain in the p-type InGaAs contact layer21 over the p-type InP clad layer. Abscissa is a distance (μm) from theportion immediately above the center of the active layers, and ordinateis a strain (%) in the p-type InGaAs layer. A dotted line in FIG. 2indicates a case, in which p-type InGaAs layer is in lattice match withInP in the flat portion, and in such case, a larger tensile strain isfound in an inclined portion immediately above the active layers. On theother hand, a solid line indicates a strain in the contact layer in acase, in which supplies of In and Ga are corrected to provide acompressive strain of 0.1% in the flat portion of the p-type InGaAslayer. The composition of the contact layer for providing a compressivestrain of 0.1% is In_(0.546)Ga_(0.454)As. Here, it is confirmed that theinclined portion immediately above the active layer and the flat portionhave strains, which are equal to or lower than a critical strainaccording to the model of J. W. Matthews and A. E. Blakeslee.

Next, a process for manufacturing the buried semiconductor in firstembodiment will be described in reference to FIGS. 4A to 4F. In thepresent embodiment, a crystal growth is conducted by a metalorganicvapor phase epitaxy (MOVPE) process, and source materials employed forthe process includes trimethylindium (TMIn), triethylgallium (TEGa),arsine (AsH₃), phosphine (PH₃), disilane (Si₂H₆) and diethyl zinc(DEZn).

First of all, as illustrated in FIG. 4A, an n-type InP clad layer 12(n=1×10¹⁸ cm⁻³, thickness: 0.5 μm) an n-side InGaAsP guide layer 13(thickness: 0.05 μm), a quantum well active layer 14 exhibiting a PLemission wavelength of 1.3 μm, a p-side InGaAsP guide layer 15(thickness 0.05 μm) and a p-type InP clad layer 16 (p=1×10¹⁸ cm⁻³,thickness: 1 μm) are consecutively grown in this sequence on an n-typeInP (100) substrate 11 (n=2×10¹⁸ cm⁻³). These layers 12 to 16 mayalternatively made of other materials such as InGaAlAs, as long as thematerial is in lattice match with InP.

Subsequently, as shown in FIG. 4B, a silicon oxynitride (SiONx) filmmask 41 is deposited on the p-type InP clad layer 16, and then a photoresist 42 is applied thereon, and a mesa etch pattern is formed in theSiONx film mask 41 by employing a known PR (photo resist) process.

Next, as shown in FIG. 4C, a mesa-etching is carried out to reach then-type InP substrate 11 through a mask of the SiON-x film 41 by a dryetching or a wet etching. The quantum well active layer 14 showed inFIG. 4B is divided into an active layer 17 and a carrier recombinedlayer 43. In such case, the carrier recombined layer 43 mayalternatively have an etched off structure. Alternatively, a patternedSiONx mask is formed on the n-type InP substrate 11, and then the activelayers may be formed by a selective epitaxial growth technology.

Subsequently, as shown in FIG. 4D, the PR process is employed again toremove the SiONx film mask 41 on the carrier recombined layer 43 whileonly a portion of the SiONx film mask 41 on the active layer 17 isprotected with a photo resist 51.

Next, as shown in FIG. 4E, block layers composed of a p-type InP blocklayer 18 (1×10¹⁸ cm⁻³, thickness: 1.0 μm) and an n-type InP block layer19 (n=5×10¹⁸ cm⁻³, thickness: 0.5 μm) are grown through a selective maskof the portion of the SiONx film 41 on the active layer 17.

Furthermore, as shown in FIG. 4F, the portion of the SiONx mask 41 onthe active layer 17 is removed, and then the whole substrate is coveredwith a p-type InP clad layer 20 (p=1×10¹⁸ cm⁻³, thickness: 2.5 μm), andthereafter, a p-type InGaAs contact layer 21 (p=1×10¹⁹ cm⁻³, thickness:0.3 μm) is grown. In such case, the p-type InGaAs contact layer 21 isgrown so as to have a compressive strain of 0.1 to 0.2% in the flatportion thereof that is sufficiently remote by 50 μm or further from theportion thereof immediately above the active layer 17. Here, the crystalgrowth of the contact layer is conducted by employing trimethylgallium(TEGa) as a source material of Ga, and trimethylindium (TMIn) as asource material of In, and the flow rates of these materials may besuitably controlled to provide a controlled strain to be applied to thecontact layer. In order to form the contact layer having a compressivestrain of 0.1 to 0.2%, the flow rates of trimethylgallium andtrimethylindium can be adjusted according to the above-described formula2 so as to obtain a composition of InGaAs within a range of fromIn_(0.546)Ga_(0.454)As to In_(0.56)Ga_(0.439)As.

After the growth, a p-side electrode and an n-side electrode are formedby employing a known process for manufacturing semiconductor laserdevices to obtain a semiconductor device. While the regions of the cladlayer 20 and the contact layer 21 immediately above the active layer 17are configured to have a U-shaped cross section of the bottom surface inFIG. 4F, the configuration is not limited thereto, and for example, analternative configuration having a flat horseshoe cross section of thebottom surface may also be adopted.

Second Embodiment

In reference to FIG. 3, another embodiment of a buried semiconductorlaser, in which the p-type InGaAs contact layer 21 in FIG. 1 is replacedwith a superlattice structure of a p-type InAs layer and a p-type GaAslayer will be described. In FIG. 3, the p-type InGaAs contact layer 31is composed of a superlattice structure of a p-type InAs layer 32 and ap-type GaAs layer 33. In the present embodiment, similarly as in firstembodiment, a growth is conducted to reach the p-type InP clad layer 20,and then an InAs/GaAs superlattice structure 31 composed of the p-typeInAs layer 32 and the p-type GaAs layer 33 is grown. In such case, it isconfirmed that the p-type InAs/GaAs superlattice structure 31 is grownto have an average compressive strain within a range of from 0.1% to0.2% in the flat portion thereof that is sufficiently remote by 50 μm orfurther from the portion thereof immediately above the active layers, sothat the flat portion and the inclined portion immediately above theactive layer 17 have strains not higher than a critical strain.

While the region of the contact layer 31 immediately above the activelayer 17 is configured to have the U-shaped cross section of the bottomsurface in the present embodiment, the configuration is not limitedthereto, and for example, an alternative configuration having a flathorseshoe cross section of the bottom surface may also be employed.

While the above-described embodiments illustrate the exemplaryimplementations of the present invention in reference to the annexedfigures, these are presented only as illustrations of the invention, andvarious modifications other than that disclosed above may also beavailable.

It is apparent that the present invention is not limited to the aboveembodiment, and may be modified and changed without departing from thescope and spirit of the invention.

1. A buried semiconductor laser, comprising: an n-type indium phosphide(InP) substrate; an active layer disposed on said n-type InP substrate;block layers bilaterally provided on both sides of said active layer; aclad layer provided over said active layer and said block layers; and ap-type gallium indium arsenide (InGaAs) contact layer provided over saidclad layer, wherein said p-type InGaAs contact layer has a compressivestrain.
 2. The buried semiconductor laser according to claim 1, whereinsaid p-type InGaAs contact layer includes a flat portion remote from aportion thereof immediately above said active layer and an inclinedportion immediately above said active layer, said flat portion of thep-type InGaAs contact layer has a compressive strain that is not higherthan a critical strain, and said inclined portion of the p-type InGaAscontact layer has a tensile strain that is not higher than a criticalstrain.
 3. The buried semiconductor laser according to claim 2, whereinsaid flat portion of the p-type InGaAs contact layer has a layerthickness of 0.3 μm, and the compressive strain of said flat portion iswithin a range of from 0.1% to 0.2%.
 4. The buried semiconductor laseraccording to claim 3, wherein said p-type InGaAs contact layer iscomposed of a superlattice structure of a p-type indium arsenide (InAs)layer with a p-type gallium arsenide (GaAs) layer, and wherein saidp-type InAs layer and said p-type GaAs layer have average compressivestrains within a range of from 0.1% to 0.2% in said flat portion.
 5. Amethod for manufacturing a buried semiconductor laser, comprising:forming an active layer an on n-type InP substrate; forming block layerson both sides of said active layer; forming a clad layer over saidactive layer and said block layer; and forming a p-type InGaAs contactlayer over said clad layer, wherein said forming the p-type InGaAscontact layer includes forming the p-type InGaAs contact layer having acompressive strain.
 6. The method according to claim 5, wherein saidforming the p-type InGaAs contact layer includes forming the p-typeInGaAs contact layer having a flat portion remote from a portion thereofimmediately above said active layer and an inclined portion immediatelyabove said active layer, said flat portion of the p-type InGaAs contactlayer having a compressive strain that is not higher than a criticalstrain, and said inclined portion of the p-type InGaAs contact layerhaving a tensile strain that is not higher than a critical strain. 7.The method according to claim 6, wherein said forming the p-type InGaAscontact layer includes forming the p-type InGaAs contact layer, in whichsaid flat portion has a layer thickness of 0.3 μm, and the compressivestrain of said flat portion is within a range of from 0.1% to 0.2%. 8.The method according to claim 7, wherein said forming the p-type InGaAscontact layer includes forming the p-type InGaAs contact layer, which iscomposed of a superlattice structure of a p-type indium arsenide (InAs)layer and a p-type gallium arsenide (GaAs) layer, said p-type InAs layerand said p-type GaAs layer having average compressive strains within arange of from 0.1% to 0.2% in said flat portion.